1. Field of the Invention
This invention relates to microprocessor based electrical apparatus, such as electrical contactors, having as inputs 120 vac signals switched by switches such as push-button stations or programmable logic controllers protected by noise suppression capacitors which produce false ac inputs when the switch is open. More particularly, the invention is directed to apparatus for detecting true ac input signals to such microprocessor based apparatus, and specifically to apparatus which rejects the false ac signals produced by the noise suppression capacitors when the input switch is open.
2. Background Information
Many types of electrical apparatus for controlling electric power, such as electrical contactors and circuit breakers, now incorporate a microprocessor. For the most part, however, such apparatus is still controlled by 120 vac signals generated by push-button stations or programmable logic controllers (PLC). Typically, the inputs of the microprocessors in these new contactors and circuit breakers have clamping circuits which clip the 120 vac signal at about five volts.
It has been found that capacitively coupled signals can create false inputs for these microprocessor based devices. For instance, U.S. Pat. No. 4,748,343 recognized that parasitic capacitance can couple false ac inputs to the microprocessor. This patent further recognized that the capacitively coupled false signal leads the ac signal and teaches the use of a zero crossing algorithm to discriminate between true and false ac inputs. The zero crossing of the unswitched line voltage is used as the reference for the zero crossings. Due to larger filter capacitors on the inputs to which the switched ac signal is applied, the zero crossings of the true ac signal lag the zero crossings of the reference. On the other hand, capacitively coupled ac signals lead not only the true ac signal but also the reference ac signal. Accordingly, U.S. Pat. No. 4,748,343 teaches a technique in which the ac input is sampled a predetermined time interval after the zero crossing of the reference ac signal. If a signal is detected at this instant, it is a false ac signal.
It is common, and especially in the case of the PLC, to provide a noise suppression network including a shunt capacitor across the switch providing the ac input signal to the microprocessor. This noise suppression capacitor capacitively couples ac to the microprocessor input when the switch is open. The larger this noise suppression capacitor is, the smaller is the angle by which the false signal leads the true ac input. In fact, as the size of the noise suppression capacitor increases, this lead can be reduced so that while the false signal leads the true ac signal, it lags the reference signal. All of this places tighter constraints on the timing of the sampling to discriminate between the true and false ac inputs. Furthermore, less expensive inputs to the microprocessors have less accurate switching thresholds which also decreases the measurement accuracy of the phase shift of the false ac signal.
There is a need therefore for improved apparatus for discriminating between true and false capacitively coupled ac inputs to the microprocessors of electrical apparatus such as contactors and circuit breakers.
More particularly, there is a need for such apparatus which can discriminate between true ac inputs and ac inputs capacitively coupled by sizable noise suppression capacitors shunting switches applying switched ac signals to the microprocessor.
There is a further need for such apparatus which can be used with less expensive microprocessor inputs which have less accurate switching thresholds.